Embodiments of axial flow machines include, for example, jet airplane engines, axial turbines, axial pumps, axial compressors, and axial fans. In the following, the invention and the underlying problem will be explained with reference to axial fans. The invention may be used with any kind of flow machines, though, and is not limited to a specific fluid either. Axial fans are used in different technical fields and serve to transport gases or gaseous suspensions through the housing surrounding the rotor, which may be a part of a system of tubing. High pressure axial fans with very large drive powers of up to 20 MW are used in flue gas desulferizing devices of large power plants, garbage incinerating plants, and wind tunnels. Axial fans of common air conditioning systems have drive powers in the order of 50 kW. Axial fans of different motor cooling systems have power ratings below this. In the lowest power rating class are for example axial cooling fans used in computers.
From aero- and hydrodynamics it is well known to force a laminar-to-turbulent boundary transition to occur at surfaces in a flow by using turbulence generators. The acceleration for forcing of the laminar-to-turbulent boundary transition is also used in aeronautics. A turbulent boundary layer may have a higher drag, but there is also a smaller tendency for stall. For instance, so-called trip wires or sandpaper are often used on gliders to create turbulent boundary layers at the wings.
A method for the improvement of the efficiency of an axial flow machine is known from the German Offenlegungsschrift 15 03 636. Here, the inner surface of the casing surrounding the rotor is roughened in order to reduce the induced resistance at the blades of axial fans. This treatment is supposed to cause the formation of a boundary layer at the inner surface in the region of the rotor or amplify an already existing boundary layer. The boundary layer serves to close the gap between the tips of the blades of the rotor and the casing. It is not the object to close the gap, though, on the contrary it is suggested to make the gap larger than normal. The roughness of the inner surface of the casing, which acts as a turbulence generator, can be effected by the application of a material that enlarges the roughness of the inner surface. Alternatively, it is possible to groove the casing along the perimeter region of the rotor and to fill this groove with a rough material. The width of the region of the casing provided with a roughness should correspond to the axial length of the blades of the rotor at their tips.
A method for the reduction of sound emission using a turbulence generator and a flow machine with a turbulence generator are also known from the French Patent 85 15 737. In this case a so-called trip wire as a turbulence generator is arranged directly on the blades of the rotor. This reduces the tendency for laminar stall of the rotating blades. This in turn reduces the sound emission and increases the usable region of the aerodynamic characteristic of the flow machine. On the other hand, the flow resistance is increased and therefore the efficiency of the flow machine is reduced.
From the German Utility Model 75 35 700 a wind guide for a fan propeller is known, which has a rotor and a casing surrounding the rotor, the casing being formed as the wind guide. In order to reduce the sound emission of the fan propeller in the presence of a relatively large gap, the inside of the casing is clad with a porous, sound absorbing material, or the whole casing is made of a porous, sound absorbing material. In the region of the tips of the rotor the sound absorbing material may be covered with a thin, non-porous protective layer, in order to prevent dirt from entering the underlying porous material. The porous, sound absorbing material is effective by taking up whirls emanating from the tips of the blades of the rotor and transforming their kinetic energy into thermal energy.
In the different flow machines different widths of the gap between the tips of the blades of the rotor and the inner surface of the casing are usual. The width of the gap negatively influences the efficiency of the flow machine as well as the technically usable region of the aerodynamic characteristic, and is therefore chosen to be as small as possible without allowing the tips of the blades to touch the inner surface of the casing. The minimum attainable values depend on the manufacturing tolerance of the axial fan and the casing, on the stiffness of the fan-casing-arrangement, as well as on the smooth running of the rotor of the axial fan. Furthermore, thermal effects must be taken into consideration, i.e. different thermal expansion rates during the operation of the fan. Gaps with especially large widths cannot be avoided in axial fans with adjustable blades for different stagger angles, since the cylindrical gap geometry is usually changed with a different setting of the blades.
High pressure axial fans of flue gas desulferizing plants, for example, have very small gap widths of 0.1% of the rotor diameter, corresponding to absolute values of 2.5 to 3.5 mm with rotor diameters typically in the range of 2.5 to 3.5 meters. This necessitates an elaborate manufacturing process, which is economical due to an increase of efficiency, though, and therefore usual. Compared to this, higher quality motor cooling fans have which are usually not less than 0.5% of the rotor diameter. Axial fans used in computers have significantly higher relative gap widths, still. This cannot be avoided in the high volume, cost effective production, since smaller widths of the gap would already present the danger of the tips of the blades coming into contact with the inner surface of the casing.
Apart from the loss of efficiency with large gap widths, significant sound emission is registered. Especially in certain frequency ranges below the blade frequency a so-called "blade-tip-whirl-noise" occurrs, which dramatically increases with the width of the gap between the tips of the blades and the inner surface of the casing. The technically usable region of the aerodynamic characteristic of an axial flow machine, i.e. the smallest still usable volume flow, is determined by the gap in the sense that the flow machine can operate at slower speeds, without instationary transport occurring.
From the book "Compressor aerodynamics" (N. A. Cumpsty, New York, 1989) it is known to extend the technically usable region of the aerodynamic characteristic through a modification of the casing surrounding the axial fan, the so-called "casing treatment". This entails bringing slits ending in the casing into the inner surface of the casing. These slits are preferably oriented axially and may be slanted with respect to,the inner surface of the casing. With this casing treatment an enlargement of the usable region of the characteristic may be obtained, but this is connected to a decrease of efficiency. Furthermore, it is disadvantageous that the casing treatment is considerably elaborate, especially when being applied to already existing machines. There is also the danger of particles clogging the slits of a flow machine with casing treatment when transporting gaseous suspensions.